Why can't I switch out of object mode?
I copied and pasted an object from a different Blend. Maybe that's why it was locked. But why is that?


If you link an object from another file, it can only be edited in that file, not the one you have added it to. What Blender is doing is making an exact copy of a portion of another .blend file, and reverse connections are not currently supported (being able to update the linked .blend). You will need to Import the mesh in order to edit it. 

Changes to the source .blend file will not be automatically visible in the linked mesh, you will need to re-link or update the mesh.